Electrodeposition of metals and alloys



March 20, 1951 J. M. BooE 2,545,566

ELECTRODEPOSITION OF METALS AND ALLOYS Filed March 11, 1945 2 Sheets-Sheet 1 0R fis) SUR/:140

mVENToR. fd/fies arl/lha mea/mv March 20, 1951 J. M. BOOE ELEcTRoDEPosITIoN oF METALS AND ALLoYs Filed 'March 11, 1945 2 Sheets-Sheet 2 INVENToR. faffzfs /arwhaae BY 025%@ @MMV Patented Mar. 20, 1951 UNITED STATES PATENT OFFICE ELECTRODEPOSITION F METALS AND ALLOYS Application March 11, 1943, Serial No. 478,750

y 2 Claims. l

This invention relates to the electric deposition of metals and alloys.

An object of the invention is to improve the methods, bath compositions and apparatus for the electrodeposition of metals and alloys.

Other objects of the invention will be apparent from the description and claims.

In the drawings:

Figure 1 is a diagram illustrating the preferred working range of anode current density for electrodeposition according to the present invention;

Figure 2 shows a plating tank and apparatus for plating the inside of a cylinder;

Figure 3 shows apparatus using two anodes and two power sources for alloy plating;

Figure 4 shows a tank plating apparatus using a single power source;

Figure 5 is a longitudinal section of an anode for alloy plating;

Figure 6 shows a modified anode feeding arrangement for alloy plating; `and l Figure 7 shows another anode feeding arrangement.

In many plating operations of the prior art a great deal of trouble has been experienced in obtaining smooth uniform deposits over extended periods of time. While the plating bath may operate satisfactorily for the rst few hours, the deposits produced by the bath may eventually become rough and useless necessitating renewal or replacement of the plating bath. While there may be several contributing factors, it is believed that the principal cause of the roughness is precipitation of fine particles from the solution. Most of these are probably fine metallic particles given off by the anode as it dissolves and metallic impurities from the anode which are not dissolved with the anode metal although some of them may also be due to metal compounds formed by reaction with one of the constituents of the plating bath such as the brightener.

The troubles are particularly marked in alloy plating such as in the deposition of silver-lead alloys. For example, a new solution for silverlead plating will satisfactorily deposit silver-lead for the rst several hours.Y However, with continued operation the deposit becomes increasingly rough and after ten to fifteen hours operation it is so rough as to be useless. Continuous or periodic filtering will not adequately keep the bath free of these fine particles originating at the anode or prevent them from reaching the cathode. On this silver plating the difficulties are not so marked but where heavier deposits are to be built up, such as those of several thousandths of an inch in thickness, these particles produce excessive roughness of the deposit.

The introduction of metal particles from the anode into the plating bath appears to result from a selective dissolution of the anode metal so that small particles or crystals of the anode metal become loosened and drop ol the anode. This is borne out by the observable crystalline surface appearance of an anode while being used in the conventional electroplating processes.

Closely associated with this crystalline condition is the tendency in some plating processes to the formation of so-called black anodes. This is perhaps the most important anode problem in large scale silver plating operations, for example. In usual prior art dissolution of silver anodes, the anode surface appears crystalline, matte or dull, and of a very light gray color. Under some conditions, however, the anode surface becomes much darker, or even black, and a fairly tenacious or adherent scum appears, Particles Yof this scum become loosened from the anodes and become suspended in the bath. The cause of these black anodes has been attributed to small amounts of impurities in the anode which are not readily soluble in the plating solution. These may amount to less than 0.05% of the anode weight and still produce this black condition.

I have found that the anode surface condition can be changed by increasing the anode current density. Various anode surface conditions are indicated in the diagram of Figure 1. With low anode current densities the anode takes on a crystalline, matte, or dull, appearance. Plating processes of the prior art operated the anodes in this range. This has resulted in metal particles falling into the solution. In this range, the current is roughly proportional to the applied voltage. ,The black anode condition, if present, occurs in this range.

As the voltage is increased, a point A is reached where the anode rathersuddenly becomes smooth and bright and is no longer crystalline in appearance. This may be accompanied by fluctuating shadows playing over the anode surface. It is also accompanied by a sudden rise in voltage at the anode without a proportional rise in current. Further increase in voltage will not proportionally increase the current density. The anode may be said to be in a semi-polarized condition. Dissolution appears to take place uniformly over the anode surface.

The semi-polarized range has as its upper limit B the current density at which the anode becomes completely polarized as evidenced by formation of an insoluble dull coating on the anode surface or generation of gas.

The type of behavior described above applies to the platable metals generally, that is, to zinc and the metals below it in the electro-chemical series, such as cadmium, iron, the tin group, lead, copper, silver, gold, the platinum group and others, as Well as to'allcy anodes. The current densities A and B, marking the limits of the semipolarized range, depend upon the anode metal as well as upon the composition of thel plating bath, the temperature and the rate of circulation or agitation of the bath.

I have found that by operating the anode in the semi-polarized range the plating diiculties associated with ilne metal particles in the bath can be eliminated and other advantagesattained as well. Not only are no particles of the anode metal released into the bath, but the impurities associated with black anodes do not form particles in 'the solution. In the lbright range apparently the oxidizing potential is sufiicient -to dissolve the impurities along with the Vsilver so that no precipitate forms at the anode.

`W ith ordinary alkalineplating baths, thebright range A-B is narrow and'occurs-at a-fairly'low current density. The current density for the bright range can be increased and the bright range broadened by theaddition of corrosive ions to the plating bath. These are ions which form highly soluble compounds Iwith the anode metal and hence promote-dissolution of the anode. For example, in the case of-a lead anode tartrate and citrate ions produce this result and hence the addition oone of their compounds, such as poprovide an anode area at least equal to the cathode area and usually -greater than the cathode area. This was-done because with smaller anode areas, under usual plating conditions, gassing takes place at the anode, causing a depletion Vof metal in the bath, `and in the case of cyanide baths, acceleration of the formation of carbonlates.

In the present invention the anode area is greatly reduced. In the case of lead anodes, for example, the anode area may be only 1/so to 11-0 that ofthe cathode and the anode current density may be 30 to 50 amperes per square foot. The small anode area also introduces mechanical advantages in plating certain shapes.

vIn summary, the following factors are combined to provide an electrodeposition process which yprevents the introduction of metal particles into the solution, operates at greater plating speed and has 190% anode current eiiiciency; l. .Operation of anode in semi-polarized or bright range.

`2. Use of corrosive ions inthe plating bath. This is particularly' applicable to alkaline baths. 3. Operation at high anode current density. 4. Anode areamuch smaller than cathode area. It is preferable to combine with the above other factors which permit a fuller realizationof the ad- Y ing solution.

vantages of the process. -One of these is the use of a brightener which does not form insoluble precipitates with the anode metal or plating bath. I have found the hydroxy-benzenes, such as pyrogallol, to be useful in alkaline plating baths generally. In an alkaline bath pyrogallol appears to take up oxygen and form a dark colored oolloid whichacts as the brightener. This brightener action takes place only in highly alkaline Ibaths, namely those with a pH of 13 or above.

Another improvement resides in the means used for introducing the anode into the plating bath. The anode area in contact With the solution'is small, and -itlmust be kept within certain Aiairly close limits "in order to preserve the semi- 'polarized `condition and to regulate the cathode current density. .I have devised a pointed anode rod which is allo-wed to rest on a support in the plating bath. If the semi-polarized condition is maintained along with fairly uniform agitation of the bath,the anode will-.dissolve .uniformlyand a sharply pointed condition is preserved andlthe anode settles so as tokeeplthe same ,area always in the solution. .'.ihis will be clearer from the more detaileddeseriptionand the .illustrations of the drawings.

The present invention -has .application `to *.the plating of metals and alloys generally, :but-particularly te `electrodeposition .iromalkaline baths. Following are examples of .the .inventionas applied to lead plating,.l'ead-silver alloy plating,.sil .ver platine and copperplating.

STRAIGHT LEAD PLATING The following procedure is suitable for the plating ofstraight lead on a base such as steel, silver, copper and other metals.

The article to be plated is rst degreased and then electrolytically cleaned in analkaline clean- After removal it is `desirable to thoroughly rinse the article in water. In case undercoating metals areapplied to the base before lead plating, a brief rinse of the plated article in Water is suiiicient.

The article is then immediately transferred to the lead plating bath of the :following composition, for example:

(a) Alkaline Zeadbath Grams per liter Potassium tartrate Potassium hydroxide 40 Basic lead acetate 42.5 (Equivalent lead content) 30 Pyrogallol 1 pH: 13.5-14, .temperature 5.0" C.

Plating is eiiected at 5G C. with a-current density of 25 'amperes per square foot of anode surface with agitation'of 30 feet per minute past the electrodes. 'The .cathode area being greater than the anode areagthe cathode current "density will be'less than 25 amperes per square Toot.

The plating apparatus is illustrated `in Figure 2 of the drawing'and comprises a'tank l a containing the plating bath lifthe'anode l2 :which is tapered to apoint 'i3 which rests on a perforated insulating plate M, ithecathode 'i5 comprising a bearing sleeve resting on plate lll, an insulating tubular -mask it surrounding the exterior of the cathode, and a circulating pump il. A covered conductor I3 rpasses vthrough mask it and connects the cathode to the negative terminal of 'a D. C. power source. The pump Il is driven by electric motor i9 and draws in plating solution at 2B .'andfforces'itvia pipe -2I `through the holes in plate I4 and up through the center of the cathode bearing shell I5.

The anode I2 comprises a rod of lead(prefer ably round) which is tapered to a point I3 at its lower end. The taper is of such length as to provide the desired anode area in the tapered section. The support for the point I3 and the s0lu tion level are adjusted to bring the surface of the solution even with the upper end of the taper. The anode rod is supported loosely by an insulating sleeve 22 above the solution so that it may settle into the solution as the tapered portion is dissolved. f

I have found that under these conditions, when the anode is operated in the bright or semi polarized condition, and the circulation or agitation of the solution around the anode is fairly Iuniform, the current is uniformly distributed over the entire tapered area so that the tapered surface dissolves uniformly from the beginning of the taper to'the tip, thus the same taper is always preserved as the anode rod settles into the solution and no particles or sections can become eaten away from the anode and drop off into the solution.

Another lead bath which is somewhat better than the above is as follows:

(b) Alkaline cyanide lead bath Grams per liter Potassium tartrate 100 Potassium cyanide (total) 145 Lead acetate 52.5 y Pyrogallol 1 pH=l3.5-l4, temperature 50 C.

With agitation of 30 feet per minute, an anode current density of 50 amperes per square foot is used with a cathode density of 5 to 25 amperes per square foot depending on its size.

While the above are the preferred conditions, given by way of example, it is contemplated that some deviation from these conditions can be made in some instances.

Other anodic dissolution producing ions may be substituted for the tartrates such as acetates, citrates, formates, malates, i. e., one which will dissolve lead to form a soluble compound.

SILVER-LEAD PLATING For plating any base metal, such as a steel bearing shell with a silver-lead alloy, the steel blank is processed in the same manner as for lead plating. A copper strike followed by a silver strike may then be applied. However, I have found that a silver strike alone may be used. In this case the cleaned steel shell is soaked in the silver strike solution for one minute and then a strike current of 300 amperes per square foot of cathode surface is applied for one minute. A suitable silver strike solution may consist of t Potassium cyanide, preferably at least 100 grams per liter Silver cyanide, 0.5 to 1 gram per liter and preferably not over 1 gram per liter The silver cyanide concentration is kept low and the potassium cyanide is kept high so as to maintain the silver ion concentration below that at which galvanic deposition of silver onto the steel will take place before current is applied. This insures that substantially all the silver which is deposited from the strike bath will be electrolytically deposited. After the silver strike, the

shell is transferred immediately to the silver-lead plating bath. This bath `may consist of:

(c) Silver-lead bath Grams per liter Silver-cyanide 120 Potassium cyanide (total) 205 (Freel potassium cyanide 145) Potassium tartrate Lead tartrate 6 Potassium hydroxide 10 Pyrogallol 0.5

Before use the plating bath is clarified with 5 to 10 grams per liter of activated charcoal and l- Y solution during plating. The lead dissolves in the plumbous or bivalent condition.

Figure 3 shows a suitable apparatus for silverlead plating. It comprises plating tank 30 containing silver-lead bath 3'I, silver anode 32, lead anode 33, cathode 35 and circulating pump 35 driven by motor 36. Both the lead anode 33 and the silver anode 32 are tapered, the tapered surfaces being proportioned to the relative areas required. Silver anode 32 rests on perforated plate 3l so that its tapered portion is located along the axis of the cathode cylinder. The lead anode 33 rests on an insulating table 38 outside the cathode. The silver anode is connected to D. C. source 39 through current regulating resistance dil and the lead anode is connected to D. C. source 5I through regulating resistance 42.

It is thus `possible to adjust both anode currents independently.

This arrangement is suitable for depositing a high silver-low lead alloy on the cathode. While the lead anode is not symmetrically located with respect to all parts of the cathode surface, the circulation insures a uniform distribution of lead ions to all parts of the plating bath. The excess of current on the parts of the cathode nearest the lead anode is small in proportion to the total current so that the silver-lead alloy is deposited uniformly as to composition, and nearly uniform as to thickness over the entire cathode area. t

The plating circuit and the amount of solution agitation, together with the relative size of the anodes and cathodes are arranged so as to provide a current density at the lead anode of 30 to 50 amperes per square foot, 'preferably 40 amperes, with a current density at the silver anode of 200 to 600 amperes per square foot, preferably 300 amperes, and a current density at the cathode of 45 to 9'0 amperes per square foot of cathode area. The lead anode has an area of about 1/10 to 1A@ of that of the cathode while the silver anode has anarea of about 1A of that of the cathode.

The preferred operating temperature is 35 to 50 C. with moderate agitation, for example 45 C. The percentage of lead and silver in the electrodeposited silver-lead alloy can be varied by adjusting the areas of the anodes under solution and the cathode current density is also con- U der solution and the solution agitation. The proportion of lead deposited will be higher with the lower cathode curent densities.

The process finds its most important use in producing silver-lead deposits containing to '7 or 8% lead but is also applicable to the production of higher lead alloys. Heavy deposits can readily be produced by this process such as those used for silver-lead lined bearings. For example, it is possible to plate an alloy containing 3 to 4% lead and of 1/8 in thickness.

While a suitable silver-lead plating bath is given in the above example, it is, of course, possible to vary the composition and proportions of the ingredients in the plating bath considerably and still obtain the desirable results of the present invention. For instance, the amount oi" silver cyanide may be varied over a considerable range as may that of the potassium cyanide. It is important, however, to have a high concentration of corrosive ion, such as tartrat-e, present. In the case of tartrate, it is also important to have considerable cyanide and/or hydroxide ion present as tartrate alone gives a rather crystalline or spongy appearance to the lead anode.

It will be noted that the above solution contains no carbonatos. The literature on silver plating almost invariably recommends the addition oi' carbonatos. I have round however, that in a silver plating solution ha" g a high concentration of silver and free potassium cya nide, the addition oi carbone' .l as potassium or sodium carbonate, will not appreciably increase the conductivity,7 of the solution. There is, however, a detrimental effect on the bright working lead anode in a silver-lead plating bath and the carbonatos greatly limit the maximum anode current density obtainable. While the advantages of the present invention may be achieved to some extent in plating baths in which carbonates are present, I prefer to avoid their use.

STRAIGHT SL/'ER PLATNG An apparatus similar to that shown in Figure 2 may be used with a silver plating bath and silver anode. One suitable plating bath may consist of:

(d) Silver bath,

Grams per liter Silver cyanide 129 Potassium cyanide (total) 205 (Free potassium cyanide 145) Potassium tartrate 10G Potassium hydroxide l Pyrogallol 0.5

The conditions are about the saine as for silverlead plating.

STRIGHT COPPER PLATNG Using the apparatus or Figure 2, copper may be deposited using a:

(e) Copper both Grains per liter Copper cyanide 155 Potassium cyanide (total) 247 (Free pot ssiuni cyanide 15) Potassium hydroxide 15 Potassium tartrate o Pyrogallol 0.5 Sorbitol 60 The pH is adjusted to 13.5 by varying the KOH present. Temperature 70% C. Plating may be pered anodes [i5 and iii and cathode il?.

COPPER-LEAD PLATNG For copper-lead plating using the apparatus of Figure 3, the following bath may be used:

(f) Copper-lead bath Grams per liter Copper cyanide 165 Potassium cyanide (total) 247 (Free potassium cyanide Potassium hydroxide 15 Potassium tartrate Lead acetate 10 Eyrogallol 0.5 Sorbitol 60 TANK PLATING Figure e shows a plating apparatus suitable `for tank plating or" miscellaneous parts. It coinprises a tanir llt containing plating bath M, ta-

The bath is agitated by a stirrer |133 driven by motor 49.

rEhe anodes rest their pointed tips on insulating supports below the solution level and slide into the solution through guiding sleeves Sil and i of insulating material. The cathode parts are suspended by hooks 52 from bus bar 53 connested to the negative terminal of a D. C. source The anode@ and f3.6 are connected to the positive terminal of the source through current regulating adjustable resistors and 56 respectively. The current can thus be readily proportioned between the anodes.

The anodes may be of the same or different composition depending upon whether pure metal or an alloy is to be deposited, the plating bath being of suitable composition.

it is also contemplated that in the arrangements of Figures 2 and 4 the anodes can be i formed of an alloy of the composition to be de- Y 73 or feeding means.

taper 64. With corrosive ions for both metals in the solution, it is possible to operate the anode with both metals working in the bright range so that uniform dissolution takes place and the same taper is preserved as the anode settles into the bath. With a silver-lead anode there is some tendency of the lead to polarize due to the high current density but this can be overcome by using sufficiently high pH solutions. The thickness of the metal coating 62 is selected to give a relative cross-section of the two metals in the proportions desired in the electrodeposited alloy.

Figure 6 shows a modiiication in which the two anodes E and 66 are clamped in an insulating yoke 67 at their upper ends and the point of anode 65 rests on table 68 under the solution and is guided by loose sleeve guide 69. apparent that dissolution of both anodes will take place at the same linear rate and hence the amount of the metals dissolved will be in proportion to their areas.

Figure 7 shows a method of introducing an anode below the surface of the plating bath. The plating tank 76 is provided with a circular aperture which is lined with a soft rubber or neoprene ring l'l through which anode H2 is fed by'a spring A stop 'i4 in the Ibath maintains the same tapered length in solution as the anode dissolves.

BRIGHTENERS Pyrogallol, a trihydroxy benzene having the formula CsHs(OI-I)a, is an exceptionally good brightener for metal plating. -The amount required does not appear to be critical and I have successfully used it from a trace up to 5 grams per liter and found the bath capable of producing very bright metal and alloy deposits in this range. For the best functioning of this brightener to avoid roughness or nodules, the bath should contain a free hydroxide content to give a pI-I of 13 or above. The preferred ratio of pyrogallol to KOH is'1z100 on a weight basis but may be varied over a considerable range. Pyrogallol belongs toa group of organic compounds (hydroxy benzenes) having Very strong reducing properties. Other compounds of this class which are also suitable as brighteners are:

Phloroglucinol (1,3,5 trihydroxy benzene) Hydroxyquinol (1,2,4 hydroxy benzene) Catechol (ortho dihydroxy benzene) Resorcinol (meta dihydroxy benzene) Hydroquinone (para dihydroxy benzene) Phenol (mono hydroxy benzene) The present invention makes possible not only improved electrodeposits of metals and alloys, but also introduces economies in the plating operation. Since the baths will operate satisfactorily for long periods of time without cleaning or ltering, `a great deal of labor is saved which would otherwise be required in cleaning the solutions. Moreover, due to the use of higher current densities plating is effected at a greater rate `and hence the number of 'plated pieces produced by a given plating bath in a predetermined length of time is increased. Moreover, since the metal dissolves at practically 100% eiciency, a saving in electric current is effected.

It will be The invention also introduces advantages in the electroplating of certain shapes such as the inside of hollow members, such as bearing shells, gun barrels and the like. Heretofore, it has been diiiicult to obtain suiicient anode surface area inside the hollow article for plating under conventional conditions. By the present invention Where the anode area is greatly reduced, the space problem is simplified since it is readily possible to insert an anode of much smaller surface area than the inside surface of the cathode to be plated and still maintain sufficient cathode current density.

While specific embodiments of the invention have been described, it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. The method of electrodepositing lead from an aqueous alkaline bath containing soluble lead salts which comprises employing a lead anode having an effective surface area less than 0.5 the surface area of the cathode to be plated and passing an electric current therethrough at an anode current density of about 30 to about 50 Iamperes per square foot of effective anode surface area, said current density being less than that 'required to polarize said anode and greater than that at which anode current density increases substantially proportional to the increase in impressed voltage.

2. The method of electrodepositing lead which comprises passing an electric current through an aqueous plating bath containing lead ions. tartrate ions and cyanide ions from a lead anode to a cathode to be plated at an anode current density of about 30 to about 50 amperes per square foot of effective anodic surface, said anode current density being less than that required to polarize said anode and greater than that at Which anode current density increases substantially proportional to the increase in impressed voltage, said lead anode having an eiective surface area about 1/10 to about 1/30 of that of said cathode.

JAMES MARVIN BOOE.

o REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS I Number Name Date 1,077,920 Stevens Nov. 4, 1913 1,306,479 Harbaugh June 10, 1919 1,461,276 Leech July 10, 1923 1,800,206 Birett Apr. 14, 1931 2,020,382 Schneidewind Nov. 12, 1935 2,079,842 Cinamon May 11, 1937 2,171,842 Barrett et al Sept. '5, 1939 2,176,668 Egeberg et al Oct. 17, 1939 CTHER REFERENCES Metal. Industry, June 26, 1942, Baier, pp. 435- Trans. Electrochem. Soc., vol. 74 (1938), pp. 246-249; vol. (1939), pp. 187-189; vol. 8l (1942), pp. y199-211. 

1. THE METHOD OF ELECTRODEPOSITING LEAD FROM AN AQUEOUS ALKALINE BATH CONTAINING SOLUBLE LEAD SALTS WHICH COMPRISES EMPLOYING A LEAD ANODE HAVING AN EFFECTIVE SURFACE AREA LESS THAN 0.5 THE SURFACE AREA OF THE CATHODE TO BE PLATED AND PASSING AN ELECTRIC CURRENT THERETHROUGH AT AN ANODE CURRENT DENSITY OF ABOUT 30 TO ABOUT 50 AMPERES 